mm/nommu: unexport do_munmap()

mm/nommu: unexport do_munmap()

do_munmap() does not take the mmap_write_lock().  vm_munmap() should be
used instead.

Link: https://lkml.kernel.org/r/20210604194002.648037-1-Liam.Howlett@Oracle.com
Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>